Petrogenesis, geochronology, and tectonic significance of granitoids in the Tongshan intrusion, Anhui Province, Middle-Lower Yangtze River Valley, eastern China

摘要

The Tongshan copper deposit in Anhui Province is a typical mid-sized skarn and porphyry type deposit in the Anqing-Guichi district along the Middle-Lower Yangtze River Valley, eastern China. The Tongshan intrusion is closely related to this mineralization. The intrusion mainly comprises rocks that are quartz diorite porphyry, quartz monzonite porphyry, and granodiorite porphyry. Plagioclase in these rocks is mostly andesine (An = 31.0-42.9), along with minor oligoclase. Biotite is magnesium-rich [Mg/ (Mg + Fe) - 0.52-0.67] and aluminum-poor (A1_2O_3 = 12.32-14.09 wt.%), and can be classified as magne-sio-biotite. Hornblende is TiO_2-poor (＜1.96 wt.%) and magnesium-rich [Mg/(Mg + Fe) ＞ 0.60], and is magnesio-hornblende or edenite. The SHRIMP zircon U-Pb age of the quartz monzonite porphyry is 145.1 ± 1.2 Ma, which corresponds to the middle Yanshanian period. Whole-rock geochemical results show that the rocks are silica-rich (SiO_2 = 60.23-66.23 wt.%) and alkali-rich (K_2O + Na_2O = 4.97-8.72 wt.%), and low in calcium (CaO - 2.61-5.66 wt.%). Trace element results show enrichments in large ion lithophile element (e.g., K, Rb, and Ba) and depletions in some high field strength elements (e.g., Nb, Ta, P, and Ti). The total rare earth element (REE) content of the rocks is low (ΣREE ＜ 200 μg/g), and they exhibit light REE enrichment [(La/Yb)_N ＞ 10] and small positive Eu anomalies (average δEu = 1.16). These mineralogical, geochronological, and geochemical results show that the intrusion has a mixed crust-mantle source. The Tongshan intrusion was formed by multiple emplacements of crustally contaminated basaltic magma generated by varying degrees of partial melting of enriched lithospheric mantle and lower crust. Hornblende thermobarometry yielded magmatic crystallization temperatures of 652-788 ℃ and an average crystallization pressure of 1.4kbar, which corresponds to a depth of approx. 4.7 km. Biotite thermobarometry yielded similar temperatures and lower pressures of 735-775 ℃ and 0.6 kbar (depth 2.1 km), respectively. The parental magma had a high oxygen fugacity and was produced in a volcanic arc setting related to subduction of the paleo-Pacific plate.